1. Technical Field
The present invention relates to a multi-optical axis photoelectronic sensor for determining light entering/light shielding state for every plurality of optical axes, and in particular, to a multi-optical axis photoelectronic sensor having a function of outputting information for checking the accuracy of the optical axes alignment between a light projector and a light receiver.
2. Related Art
The light projector and the light receiver of the multi-optical axis photoelectronic sensor have a configuration in which a plurality of light emitting elements or light receiving elements is arranged in a line along a longitudinal direction within a long case body, and are arranged facing each other with the position and the direction of each optical axis aligned. In an operation, each optical axis is made effective by turns to light the light emitting element of the effective optical axis, and the light receiving quantity of the corresponding light receiving element is measured. The light entering/light shielding of each optical axis is determined by matching the light receiving quantity obtained for every optical axis with a predefined threshold value (hereinafter referred to as “light entering threshold value”).
When installing such a multi-optical axis photoelectronic sensor, worker needs to check that an optical axis on the light projector side and an optical axis on the light receiver side are aligned. In this regard, a sensor having the function of displaying information indicating the accuracy of the optical axes alignment has been conventionally developed.
For instance, Japanese Unexamined Patent Publication No. 11-345548 describes obtaining a proportion of the number of optical axes, whose light receiving quantity is greater than a constant threshold value, with respect to the total number of optical axes, and displaying such proportion in a bar graph using a plurality of indication lights. Japanese Unexamined Patent Publication No. 2002-124169 describes dividing a light receiving element group into a plurality of groups and providing a display unit for every group, and lighting the display unit corresponding to a group whose light receiving quantity is greater than a reference voltage.
Japanese Patent Publication No. 3724397 describes measuring, based on the sequential lighting of each light emitting element, the light receiving quantity of the light receiving element corresponding to the lighted light emitting element, and displaying, in a bar graph or the like, a minimum value (hereinafter referred to as “minimum light receiving quantity”) of the light receiving quantities, which are obtained while the measurement on each light receiving element makes one round.
FIG. 14 schematically shows a relationship between an angular shift amount between the optical axis on the light projector side and the optical axis on the light receiver side, and the light receiving quantity. With 0 degree defined as a complete match in the directions of each optical axis, the angular shift amount is expressed by a relative angle of one optical axis with respect to another optical axis.
In the figure, the light receiving quantity greater than the light entering threshold value is obtained in a range the relative angle is between θ1 and θ2. The light receiving quantity increases as the shift of the optical axis becomes smaller. Although not shown, a relationship similar to that shown in FIG. 14 may be realized between the positional shift amount of each optical axis and the light receiving quantity.
Therefore, the light receiving quantity is desirably made as high as possible to stably determine the light entering/light shielding for every optical axis. In the inventions described in Japanese Unexamined Patent Publication Nos. 11-345548 and 2002-124169, however, obtaining a light receiving quantity greater than the light entering threshold value is taken as a criterion of optical axis alignment, the work might be terminated without being able to secure a degree of margin with respect to the light entering threshold value.
In the invention described in Japanese Patent Publication No. 3724397, a minimum light receiving quantity of the light receiving quantities retrieved from each light receiving element is displayed, enabling an adjustment to be made to obtain the light receiving quantity having margin of a certain extent with respect to the light entering threshold value. However, in the invention described in Japanese Patent Publication No. 3724397, if the detected minimum light receiving quantity exceeds a displayable range, whereafter it becomes difficult to judge to what extent the light receiving quantity can be increased. Furthermore, the completion of the optical axis adjustment at sufficient accuracy may not be assured even if the light receiving quantity is increased to the displayable range since the measureable light receiving quantity differs depending on the distance between the light projector and the light receiver, the peripheral environment, or the like.